1. Field of the Invention
The present invention relates to a fiber-reinforced plastic composite material for fastening pallets to be used to transfer printed circuit boards and also to a process for producing the same. The composite material has high mechanical strength, good antistatic property, good heat resistance, and good machinability. It can be colored in any light color.
2. Description of the Prior Art
The mounting of semiconductor parts on a printed circuit board is usually accomplished by means of an automatic soldering machine. For semiconductor parts to be placed at right soldering positions, it is necessary to carry the printed circuit board to a desired position in the machine, to stop it there, and fix it there accurately, without causing warpage. A known way to accomplish this is to transfer a printed circuit board by the aid of a pallet to which it is fastened. (See Japanese Patent Publication No. 20786/1980.)
This type of fastening pallet for transfer is usually made of a metal frame (of stainless steel, aluminum, or titanium) and a means to secure a printed circuit board. Being of general-purpose type, it can be applied to a large variety of printed circuit boards but, on the other hand, it needs fine adjustment for fastening position according to the specific printed circuit board to be transferred. This adjustment is troublesome and inefficient. In addition, the recent trend toward the miniaturization and weight reduction of electronic machines and equipment has evoked the need for printed circuit boards of varied shapes and for flexible printed circuit boards. Under these circumstances, it turned out the conventional general-purpose fastening pallets do not meet requirements for efficient operation.
To cope with this situation, there has been proposed a new fastening pallet that can be applied to flexible printed circuit boards efficiently. It is made of metal plate formed in conformity with the configuration of a particular printed circuit board. (See Japanese Patent Laid-open No. 54991/1990.) However, this pallet suffers the disadvantage of being heavy and having a high thermal conductivity (because it is made of metal). Thus, it easily gets hot during soldering, which causes the sticking of solder.
To solve this problem, there has been proposed a fastening pallet of special-purpose type which is made of fiber-reinforced plastic laminate formed in conformity with a particular printed circuit board. This laminate is prepared by heating and pressing from a half-cured prepreg of nonwoven fabric of glass fiber impregnated with an epoxy resin. The fastening pallet of this kind, however, has the disadvantage that the composite resin is subject to deterioration due to heat to which it is exposed repeatedly during its use and this deterioration leads to delamination.
Moreover, the fastening pallet made of plastic composite material is liable to accumulate static charge during soldering, and there is an extreme case in which the accumulated static charge breaks electronic parts on the printed circuit board. To avoid this trouble, an attempt has been made to incorporate the composite material with carbon black, thereby rendering the composite material electrically conductive (about 10.sup.7 .OMEGA./.quadrature.) and permitting the leakage of static charge.
Unfortunately, carbon black incorporated into the plastic composite material falls off or fouls electronic parts (resulting in short-circuiting) when rubbed during use. In addition, carbon black makes the pallet black and the black color brings about heat loss at the time of heating and prevent the coloring for good appearance.
There is another way of rendering the plastic composite material electrically conductive by incorporating it with metal fiber or metal powder. (See Japanese Patent Laid-open No. 31346/1991.) Metal fiber does not disperse uniformly when the composite material undergoes laminate forming. This leads to uneven electrical conductivity. Metal powder is liable to precipitate (due to its higher specific gravity) and hence does not distribute uniformly. This also leads to uneven electrical conductivity. Moreover, metal powder involves the danger of dust explosion in the manufacturing process.